The world’s largest animals have been hiding something. The bodies of the giant rorqual whales—including the blue, fin and humpback—have been regularly displayed in museums, filmed by documentary makers, and harpooned by hunters. Despite this attention, no one noticed the volleyball-sized sense organ at the tips of their lower jaws. Nicholas Pyenson from the Smithsonian Institution is the first, and he thinks that the whales use this structure to coordinate the planet’s biggest mouthfuls.

The rorquals sieve tiny prey from the water with a unique hunting technique called lunge feeding. They surge forwards, open their mouths and swallow everything in front of them. This seemingly simple tactic is one of the most extreme in the animal kingdom. In one move, a lunging fin whale can engulf a volume of water that’s bigger than its own body. Its bigger cousin – the blue whale – can swallow half a million calories in one gulp.

Here’s what happens in slow-motion prose. A hunting rorqual detects the movements of their prey with pressure-sensitive whiskers on the underside. It accelerates to high speed and opens its mouth to almost a right angle. The two halves of its lower jaw – the largest bones that ever existed – are connected to its skull by flexible joints, and their tips aren’t fused together. This allows them to swing outwards, widening the mouth.

As water floods in, the mouth balloons out. Blubbery pleats that run along the underside of the whale’s throat allow it to expand without tearing (the name ‘rorqual’ comes from the Norwegian for ‘furrow whale’). The tongue (which, in a blue whale, is the size of an elephant) inverts into a large flattened sac, partly retreating through the floor of the mouth. This creates more room. In just a few seconds, the whale has transformed from “a cigar shape to the shape of an elongated, bloated tadpole”.

Finally, the whale closes its gigantic mouth. The pleats contract and the tongue pops back into shape, pushing the engulfed water against bristly plates of baleen hanging from the roof of the mouth, and sieving out any tasty morsels.

Discovering a new organ

Pyenson thinks that the new sense organ helps to coordinate this process. Together with Bob Shadwick, he discovered the structure by dissecting a dozen fin and minke whales, and by placing the jaws of one specimen in medical imaging scanners. “We had no idea what to make of it when we first saw it,” says Pyenson. “It’s a bit messy and gooey in life. Imagine a gel-filled balloon-like structure lodged between two telephone poles.”

The organ was mentioned by Paul Brodie in 1993, but he interpreted it as a ball-and-socket joint. Pyenson disagrees. The gel around the organ contains many blood vessels and nerve endings, which come from a structure in the jawbone that was once the tooth socket of the first incisor, back when these whales still had teeth. These traits mark it out as a sense organ, and Pyenson thinks that it helps the whales to control the movement of their jaws during their titanic lunges.

Lunge-feeding isn’t a passive process, where the incoming water inflates the mouth like a parachute. If that was the case, simulations show that either the whale’s mouth would experience “catastrophic blow-out”, or the water would just rebound back out. Obviously, that doesn’t happen—the oceans aren’t littered with ruptured rorquals. Instead, the whale actively uses muscles in it mouth to push water forward during a lunge. This seems counterintuitive, but it smoothes out the drag forces acting upon the mouth, and prevents prey from clogging up the baleen.

To coordinate this, the whale needs to gauge what’s going on in its mouth. Nerves in its pleats certainly help, but the newly discovered organ provides the clincher. It sits in a spherical cavity at the front of the mouth, nestled snugly between the disconnected halves of the lower jaw. It also rests on top of a Y-shaped piece of cartilage, which extends back along both sides of the jaw .It’s in a prime position, connected up to the hard and soft parts of the whale’s mouth, and wired into to its brain.

During a lunge, the organ picks up signals from the jawbones, which compress it as they swing outwards. It also gets signals from the Y-shaped cartilage, which bends as the mouth expands. “A sense of stretch isn’t new, but a gross organ devoted to this sense is unique,” says Joy Reidenberg, a whale anatomist who appears on Inside Nature’s Giants. “It’s very exciting work!”

Secrets still untold

This organ is unique to the rorquals. Other baleen whales, like the bowhead and right, don’t have it. This means that it evolved in conjunction with the lunge-feeding lifestyle, or just before it. Either way, it was part of the adaptive apparatus that allowed these animals to grow enormous on a diet of tiny.

Erich Fitzgerald, who studies whale evolution at Museum Victoria, says that the next step is to get some data on how the organ is used during feeding. For example, why do the nerves running into the organ only come from one of the two jawbones, making it the only asymmetric part of the rorqual’s entire body? No one knows, but Pyenson notes that rorquals often roll to one side when they gulp. Perhaps whales with nerves coming in from one side might prefer to feed on that side. “We’ll admit that it’s a bit suggestive and speculative,” he says.

It might be surprising that such familiar animals still hold secrets, but there is much we don’t know about the giant whales. “Whale anatomy is really an opportunistic venture,” says Pyenson. “Even if you are lucky enough to find a carcass cast ashore in decent condition, you may not have the tools at your disposal to investigate everything properly. I wouldn’t be surprised if more strange tissues and organs were discovered.”

Fitzgerald agrees. “We still have so much to learn about the basic biology of some of the most storied, controversial, popular and enormous animals on Earth,” he says. “There is still much to learn from investigating the fundamentals of anatomy and natural history—sciences that are as relevant and dynamic today as they were in the 18th century. The great days of zoological exploration are clearly not yet done!”

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